A Rotating-Tip-Based Mechanical Nano-Manufacturing Process: Nanomilling
© The Author(s) 2010
Received: 25 March 2010
Accepted: 17 May 2010
Published: 25 June 2010
We present a rotating-tip-based mechanical nanomanufacturing technique, referred to here as nanomilling. An atomic force microscopy (AFM) probe tip that is rotated at high speeds by out-of-phase motions of the axes of a three-axis piezoelectric actuator is used as the nanotool. By circumventing the high-compliance AFM beam and directly attaching the tip onto the piezoelectric actuator, a high-stiffness arrangement is realized. The feeding motions and depth prescription are provided by a nano-positioning stage. It is shown that nanomilling is capable of removing the material in the form of long curled chips, indicating shearing as the dominant material removal mechanism. Feature-size and shape control capabilities of the method are demonstrated.
Tip-based creation of nano-scale features by mechanical removal has been considered since the 1990s. Motivated by the agility, geometric capability, and wide-range material applicability of mechanical removal processes, surface characterization instruments such as scanning tunneling microscopes (STMs) , atomic force microscopes (AFMs) , and nano-indenters  have been applied to create nano-scale features on various materials. Using AFMs at the force levels significantly higher than those used for topography measurements, simple features such as lines and pockets have been created on semiconductor , metal , and polymer/photoresist  surfaces through scratching or indenting. Various applications of mechanical removal at the nano-scale has been identified, including shaping of mask layers for lithography , fabrication of single-electron transistors (SETs) , and creation of Coulomb blockades, quantum point contacts, quantum nano-dots, quantum wells, and nano-wires [8–10].
Although the basic capability of AFM-based surface modification has been established, its wide-range of applicability as a viable nanomanufacturing technique has been hindered by several issues. First, the material removal mechanism is dominated by ploughing (i.e., plastic deformation) rather than shearing (i.e., removal of material in the form of a chip). This causes accumulation, rather than removal, of the material around the created features (i.e., ridge formation) [4, 6, 11]. Second, the throughput of the process is low due to the limited removal speeds, large force requirements, and associated rapid tool wear . Third, the depth of removal is dictated indirectly by prescribing the force by monitoring the deflection of the relatively low-stiffness AFM cantilever. Since the material removal force depends on factors such as material (surface) properties , probe geometry , and removal conditions (e.g., removal speed) , the resultant removal depth cannot be well-controlled through indirect prescription of depth using force monitoring. While several approaches have been proposed to address these issues, including the use of high-stiffness cantilevers , vibrating cantilevers [8, 12], and vibrating workpiece  or diamond tips (to reduce wear) , significant process improvements that will make tip-based mechanical material removal as a fully controllable nanomanufacturing technique have not yet been realized.
In this letter, we present a tip-based mechanical nanomanufacturing process, referred to here as nanomilling, in which an AFM tip rotated at high frequencies is used as the nanotool to remove the material in a manner similar to that of the conventional milling process. The rotary motions are obtained by out-of-phase motions of the axes of a three-axis piezoelectric actuator that hosts the AFM tip. By assembling the tip directly onto the actuator, and thereby eliminating the high compliance arising from the AFM-beam connection, a high-stiffness configuration is realized. The feeding motions in three directions, which define the overall geometry, are obtained by moving the sample using a nano-positioning stage. The combined use of high-stiffness tip and nano-positioning stage enables direct prescription of feature depth and shape. As such, the nanomilling process has the potential to advance tip-based nanomanufacturing by enhancing the shape capability and accuracy, increasing the removal speed, and reducing the removal forces and nanotool wear.
Description of the Nanomilling Process
For the out-of-plane configuration (see Fig. 1b), the nanotool is rotated in a plane perpendicular to the surface. In this case, the prescribed depth can be changed by varying the nanotool-path diameter along the sample-surface normal. Combined motion of nanotool rotation and feeding causes a portion of material (the shaded region in Fig. 1b) to be removed at each revolution of the nanotool. The amount of material removed per revolution (the size of the shaded region) depends on the prescribed feed.
Experimental Setup and Procedure
The nanomilling process is performed within the nanomilling testbed through the following procedure: First, the sample surface is located through a nanotool-sample contact algorithm using the nanotool cantilever. The piezo shaker placed under the sample is vibrated at the first resonant frequency of the nanotool cantilever (determined a priori through LDV measurements) with a sub-nanometer vibration amplitude. The sample is then slowly approached to the tool using the nanopositioning stage, while measuring the vibrations of the tool cantilever using the LDV. A contact between the tool and the sample surface causes a significant (an order of magnitude or more) increase in cantilever vibrations at the first resonant frequency and thus, can be detected. The (unwanted) inclination of sample surface is then measured and compensated. Second, the nanotool is rotated with selected frequency using the piezoelectric actuator. And third, the nanopositioning stage is used to prescribe the depth and feeding motions to create a particular feature.
Results and Discussion
Conclusions and Future Work
In summary, this letter demonstrated the viability of a nanomanufacturing technique, nanomilling, that uses piezoelectrically induced rotary motions of a nanotool (AFM tip) to create nano-scale features by mechanical material removal. Two configurations of the nanomilling process that use in-plane and out-of-plane elliptical nanotool rotations were described. The components of the nanomilling testbed and the procedure for the nanomilling were then outlined. The material removal in nanomilling was observed to be dominated by shearing the material in the form a (continuous/curled) chip. It is shown that the nanomilling process possesses a high level of geometric control, including direct prescription of the feature depth (due to the high nanotool-stiffness), creation of different feature widths in a single pass, and fabrication of shapes with complex geometries. It is concluded that, although further improvements are needed to increase the process efficiency and accuracy, the nanomilling process has a potential to become a controllable nanomanufacturing process for fabricating nano-scale features with complex geometries.
It is expected that, when compared to AFM scratching, the nanomilling process will yield lower forces, reduced nanotool wear, and improved feature quality. The future work will include an in-depth analysis of the effect of nanomilling parameters on the material removal mechanism, feature quality, and nanotool life. The nanomilling parameters of interest are the nanotool shape, nanomilling orientation, motion shape, frequency, and feed rate. The future work will enable identifying the optimal nanomilling parameters that will yield improved feature quality and minimum tool wear, thereby optimizing the overall throughput of the nanomilling process.
This work was supported in part by the National Science Foundation award CMMI-0602401 (Ozdoganlar).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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